Week 4 - Gamma Knife Flashcards
Explain the differences between radiosurgery and radiotherapy
Radiosurgery is a single dose of radiation (12-150 Gy), with ablative intent which requires a defined target that is less than 3-4cm.
Radiotherapy is delivered in small doses (1.8-3 Gy), with mitotic cell death in dividing cells as the goal. Often used with margins to treat regions and can be used for large volumes.
Discuss some clinical indications for radiosurgery
Most cases, tumour volumes must be small and unable to be treated with surgery - meningioma, brain mets, trigeminal neuralgia, essential tremor, acoustic neuroma
Explain how the Gamma Knife operates
GK is a non-invasive alternative to surgery where 1 high dose of radiation is delivered to maximise tumour injury and minimise normal tissue injury. Radiation source used is Colbalt-60 with a half-life of 5.26 years
Explain how motion management is employed for GK
Real-time motion management is employed using a system consisting of an infrared stereoscopic camera with a set of reference markers fixed to the GK head support and a reflective marker on the tip of the patient nose. Patient is monitored throughout treatment.
if the patient is out of tol by 1.5mm, they are ejected out the bore
List 5 reasons behind pre-operative SRS
- Improves target delineation
- Sterilisation of surgical bed
- Lower dose
- Reduced radio necrosis
- Shorter overall treatment
List 5 inclusion criteria for SRS
- +18 years ECOG 0-2
- Known malignancy
- At least one brain met requiring surgical resection which has not had previous RT
- Gross total resection must be reasonably expected
- Max lesion size 5cm, minimum size 2cm
List 5 exclusion criteria for SRS
- Surgery unable to proceed within 7 days of SRS
- Unable to undergo MRI with contrast
- Pregnant
- Prior SRT to treating lesion
- Presence of leptomeningeal disease outside anticipated resection zone
Explain 4 disadvantages of GK
- Requires a lot of resources
- Small specialised team not widely available in Australia
- Many contra-indications preventing patients from receiving GammaKnife
- Requires additional imaging (MRI/angiogram)
Describe the gammaknife collimator system
8 sectors with 24 sources per sector (192 sources)
Circular 4, 8, or 16mm collimator
What is gamma knife safety philosophy?
The patient positioning system and machine are seperate so that is an error or malfunction occurs in one component it shouldn’t lead to patient injury based they will be taken to a safe state
List 3 cases when a frame would be employed
- Functional cases - trigeminal neuralgia
- Vascular cases - AVM
- Long treatment time > 45mins
What is the role of MRI in gamma knife?
MRI with contrast to delineate target
can be done prior, or on the day of treatment and planning
Gamma Knife prescription for trigeminal neuralgia
80 Gy @ 100% 1#
30 minutes treatment time
Discuss a non-malignant disease treated with GammaKnife
Thalamotomy - lateral portion of Ventura intermediate nucleus of thalamus treated to 130-150 Gy
Patients must be unfit for deep brain stimulation and have a disabling tremor
frame only treatment
- functional cases eg. trigeminal neuralgia, tremor
- vascular disease eg. arteriovenous malformation (AVM)
- longer treatment > 45 mins
MRI in gammaknife
- all patients require an MRI scan (with contrast unless contraindicated eg. allergy, pacemaker)
- can be done prior to, or on the day of planning/treatment
- key role in MRI is target delineation
radiosurgery thalamotomy - patient selection
- identified at movement disorder clinic
- disabling tremor
- unfit for deep brain stimulation (DBS)
- refractory to medical treatment
radiosurgery thalamotomy - contra-indications
- active systemic lupus erythematosus (SLE)
- scleroderma or similar collagen vascular disease
- ataxia telangiectasia
what is gamma knife
- non invasive alternative to open surgery
- single high dose (12-130Gy)
- maximise tumour injury
- minimise normal tissue
DSA in gamma knife
- digital subtraction angiography
- best practice requires all AVM patients to have an angiogram performed
- done same day as planning/treatment to effectively localise imaging to the frame coordinates
- aids in target delineation
clinical indications for SRS - meningioma
most tumours are slow growing but some can grow quickly
clinical indications for SRS - brain mets
- avoids complications associated with WBRT
- improved QoL
- dose 15-22Gy @ 45-85% in 1#
- > 8cc 24Gy @ 50% 3# or 25-30Gy @ 50% 5#
clinical indications for SRS - trigeminal neuralgia
- one of the most severe pain syndromes any person can face
- when medical management fails patients require surgical intervention
- radiosurgery is the least invasive usually older patients
- dose 75-90Gy @ 100% 1#
clinical indications for SRS - parkison’s
- identified at movement disorder clinic - trembling tremor
- unfit for deep brain stimulation (DBS)
120-130Gy @ 100% 1# - thalamotomy
clinical indications for SRS - acoustic neuroma
- slow growing, benign usually develop from the vestibular portion of the 8th cranial nerve
- symptoms - unilateral hearing decline
<5cc dose 12-13Gy @ 50% 1#
>5cc dose 25Gy @ 50# 5#
clinical indications for SRS - AVM
- goal is complete obliteration
- reduce lifetime risk of haemorrhage to 1% or less
<4cc 20-22Gy @ 50% 1#
clinical indications for SRS - pituitary lesions
- role in functioning and non-functioning pituitary tumours
- gamma knife surgery is an effective and well-tolerated management strategy for the vase majority of patients with recurrent or residual non-functioning pituitary adenomas
- gamma knife offers a high rate of tumour control and a reasonable rate of endocrine remission in patients with cushing’s disease
functioning dose 18-24Gy @ 50% 1#
